Freight containers are typically of rectangular box shape, and are produced in various standard lengths. The common standard lengths are 20, 40,44 1/4, 45 and 48 feet. Each container has a flat rectangular base with corner portions and is normally fitted with corner fittings providing support and means for lifting and interlocking.
In the past, freight containers have been transported on railway flat cars in a single layer and in lengths not exceeding 89 feet, which is the conventional length restriction on individual cars. In recent times, however, with a view to making freight transportation more economic, new car designs have been developed which permit the stacking of freight containers two high. These designs have been successful and various rail systems have now succeeded in expediting service by running double-stack container trains.
The typical approach to the problem of double-stacking is to design a car with a well to accommodate a 40 foot container, (or alternatively two 20 foot containers end to end) and to stack an upper container above it. The upper container may be 40 or 45 feet long and relies on the lower container for vertical support. The lower container may also provide horizontal support through an interlocking device. In cases in which the upper and lower containers are of different standard lengths, for example where the upper container is longer than the lower container, then the upper container must be fitted with additional corner fittings at the 40 foot position, i.e. to engage the fittings of the lower container, in order to receive vertical and possibly horizontal support. This considerably complicates the container structure.